Alice in Wonderland syndrome (AIWS), also known as Todd's syndrome or dysmetropsia, is a neuropsychological condition that causes a distortion of perception. People may experience distortions in visual perception of objects, such as appearing smaller (micropsia) or larger (macropsia), or appearing to be closer (pelopsia) or farther (teleopsia) than they are. Distortion may also occur for senses other than vision.

The cause of Alice in Wonderland syndrome is currently unknown, but it has often been associated with migraines, head trauma, or viral encephalitis caused by Epstein–Barr virus infection. It is also theorized that it can be caused by abnormal amounts of electrical activity, resulting in abnormal blood flow in those parts of the brain which process visual perception and texture.

Although there are cases of Alice in Wonderland syndrome in both adolescents and adults, it is most commonly seen in children.